I have met more than one atheist who became more open about their atheism because of 9/11. I wasn't shy about telling people I was an atheist, but I didn't read books on it or go to meetups or websites until 9/11. Afterward, I found the Bush Administration's reference to the Crusades and to religious reasons for fighting back very ironic. They didn't dial back their religiosity at all in the face of that negative example of what religion can do. They actually upped it, but not the country. I think turning to prayer to overcome other people who turn to prayer had to stick in the craw of at least a few believers.
Anywho, I discovered this video in which Sam Harris talks about how he was affected too: